Apparatus and methods for separating, detecting, and measuring trace gases with enhanced resolution

ABSTRACT

Apparatus and methods for sorting and detecting trace gases which undergo ion-molecule reactions, trace ions being formed in a reactive gaseous medium and being analyzed in a nonreactive gaseous medium. The ions are classified in accordance with their velocity in an electric drift field.

United States Patent David 1. Carroll Lantana;

Martin J. Cohen, West Palm Beach; Roger F. Wernlund, Lake Worth, allotFla. 780,851

Dec. 3, 1968 Dec. 7, 1971 Franklin GNO Corporation West Palm Beach, Fla.

Inventors Appl. No. Filed Patented Assignee APPARATUS AND METHODS FORSEPARATING, DETECTING, AND MEASURING TRACE GASES WITH ENHANCEDRESOLUTION 22 Claims, 3 Drawing Figs.

U.S. Cl 250/419 TF, 250/41.9 G

Int. Cl l-I0lj 39/34i Field of Search 250/41.9 G,

SYNC PULSER References Cited UNITED STATES PATENTS 11/1956 Washbum250/41.9 (1) 10/1965 Fox eta1...... 250/43.5 X 5/1966 Fite et al...250/41.9 ISB 1/1968 Gregory 324/33 Primary Examiner-James W. LawrenceAssistant Examiner-A. L. Birch Attorney-Raphael Semmes ABSTRACT:Apparatus and methods for sorting and detecting trace gases whichundergo ion-molecule reactions, trace ions being formed in a reactivegaseous medium and being analyzed in a nonreactive gaseous medium. Theions are classified in accordance with their velocity in an electricdrift field.

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Milliseconds MARTIN J. COHEN ROGER F. WERNLUND ATTORNEY APPARATUS ANDMETHODS FOR SEPARATING, DETECTING, AND MEASURING TRACE GASES WITHENHANCED RESOLUTION BACKGROUND OF THE INVENTION This invention relatesto apparatus and methods of ion-classification and more particularly isconcerned with improving the resolution of measurements performed upontrace gases which undergo ion-molecule reactions.

The copending application Ser. No. 777,964, of Martin J. Cohen, David I.Carroll, Roger F. Wernlund, and Wallace D. Kilpatrick, filed Oct. 23,1968 and entitled Apparatus and Methods for Separating, Concentrating,Detecting and Measuring Trace Gases, discloses Plasma Chromatographysystems involving the formation of primary or reactant ions and thereaction of the primary ions with molecules of trace substances to formsecondary or product ions, which may be concentrated, separated,detected, and measured by virtue of the difference of velocity ormobility of the ions in an electric field. The primary ions may beproduced by subjecting the molecules of a suitable host gas, such asair, to ionizing radiation, such as beta rays from a tritium source,corona from a multipoint or wire array, electrons produced byphotoemission from a cathode, etc. The primary ions are then subjectedto an electric drift field, causing them to migrate in a predetermineddirection through a reaction space into which the sample or trace gas isintroduced. The resultant collisions between primary ions and the tracegas molecules produce secondary ions of the trace gas in much greaternumbers than can be produced by mere electron attachment, for example,to the trace gas molecules. The secondary ions are also subjected to theelectric drift field and may be sorted in accordance with their velocityor mobility. The specific systems of the said copending applicationemploy ion shutter grids or gates for segregating the ion species inaccordance with their drift time. The pressure of the gas in the PlasmaChromatograph cell is maintained high enough (preferably atmospheric) toensure that the mean free path length of the ions is very much smallerthan the dimensions of the cell. 7

A possible limitation upon the Plasma Chromatography technique disclosedin the said copending application results from the fact that theion-molecule interactions may not stabilize" or reach completion. Theapparent mobilities measured may be the result of ion species changingidentity one or more times during their transit through the velocityanalysis region of the drift cell. The apparent mobilities measuredunder these conditions are a rapidly varying function of traceconcentration and cannot always be uniquely identified with the tracematerial. Moreover, it appears that the additional interactions whichoccur in the analysis region are due to the trace material itself,rather than to the carrier gas.

BRIEF DESCRIPTION OF THE INVENTION It is accordingly a principal objectof the invention to provide improved apparatus and methods forseparating and detecting molecular quantities of trace substances withgreater resolution and reliability than has heretofore been possible,and to quench undesirable ion-molecule reactions.

Briefly stated, preferred embodiments of the apparatus and methods ofthe invention are concerned with Plasma Chromatography systems whichinvolve the formation of positive or negative ions by reactions betweenthe molecules of trace substances and primary ions. The secondary ionsare separated and detected in a drift cell by utilizing the differencein velocity or drift time of ions of different mass in an electricfield. In order to obtain the improved results of the invention, ionformation occurs in a reactive gaseous medium, and ion drift velocity(mobility) measurements are performed in a nonreactive gaseous medium.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be furtherdescribed in conjunction with the accompanying drawings, whichillustrate preferred and exemplary embodiments, and wherein:

FIG. I is a longitudinal sectional view, somewhat diagrammatic,illustrating a trace gas detector system of the invention;

FIG. 2 is a similar view illustrating a modified Plasma Chromatographycell which may be employed in the invention; and

FIG. 3 is a graph illustrating a typical output curve obtained by virtueof the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. I, PlasmaChromatography cell 10 in accordance with the invention may comprise agastight envelope l2 enclosing a series of electrodes, which may be ofplane parallel geometry, for example. Principal electrodes I4 and 16 maybe arranged adjacent to opposite ends of the envelope, which may be acircular cylinder. When the apparatus is used to detect negative ions,as will be assumed for example, electrode 14 will be a cathode andelectrode 16 an anode. When the apparatus is used to detect positiveions, the polarities will be reversed. As described in the saidcopending application, the Plasma Chromatography cell preferablyincludes a pair of shutter grids or ion gates 18 and 20, each of whichcomprises two sets of interdigitated parallel wires, alternate wires ofeach grid being connected together to form the two sets. Cathode 14 orthe region of the envelope near this electrode is pro vided with anionizing source, which may be of the type mentioned previously. Anode 16may be a collector plate connected to an output device, such aselectrometer 22, which may be Cary Instruments Model 401 (vibratingreed) type with current sensitivity of 10" ampe'res at a time constantof 300 milliseconds.

An electric drift field is provided between the principal electrodes I4and 16. In the form shown the source of the drift field is a chain ofbatteries 24, the negative end of the chain being connected to thecathode I4 and the positive end to ground. Anode I6 is connected toground through the input circuit of the electrometer 22. Alternatively,a resistor chain voltage divider may be employedin conjunction with abattery connected across the chain. Taps on the chain are connected to aseries of guard rings 26 spaced along the length of the envelope 12,which maintain the uniformity of the drift field.

Adjacent elements of each shutter grid 18 and 20 are normally maintainedat equal and opposite potentials relative to a grid average potentialestablished by the battery chain 24. Under these conditions the shutteror gate is closed to the passage of electrically charged particles.Potential sources which provide the equal and opposite potentials justreferred to may be considered to be part of grid drive circuits withinblocks 28 and 30 entitled Sync Pulser and Delayed Pulse." The componentsof these blocks are effective to drive the adjacent elements of eachshutter grid to the same potential, the grid average potential, atpredetermined instants, alternate grid wires being shown connected tothe battery chain by re sisters 32 and 34 to establish the grid averagepotentials.

A sample comprising a suitable host or carrier gas, such as air,carrying an appropriate gaseous trace substance, such as triethylphosphite, for example, flows into the envelope by means of a gas inletpipe 36 at one end of the envelope. Inlet pipe 36 may terminate in anopening through the cathode I4, which may include an ionizing source,such as tritium foil, indicated at 38. An outlet pipe for all gas withinthe envelope is indicated at 40 and may be partially concentric withinlet 36. Portions of the cathode may be porous to permit gas to flowtherethrough. A nonreactive or inert buffer gas, such as nitrogen,enters the envelope through an inlet pipe 42 at the opposite end of theenvelope. This gas, which may be termed the ion velocity sorter gas,fills up most of the cell, and in particular, the region from the firstshutter grid 18 to collector electrode or anode 16. Any suitable sourcesof flow pressure, such as a fan or pump, may be employed to move thegases through inlets 36 and 42 into the envelope.

In the region between the cathode I4 and first grid 18, closely adjacentto the radioactive cathode 14, primary ions of the reactive carrier gas,or one or more of the main constituents thereof, such as oxygen, areformed under the influence of the ionizing source at this region. Forexample, negative oxygen ions may be formed at cathode 14, as by directattachment of electrons to the oxygen molecules of the air host gas, thesample being subjected to beta rays produced by the tritium foilcathode. The primary ions drift toward the anode 16, and in the reactionspace between the cathode l4 and the first shutter grid 18 the primaryions encounter other molecules. A majority of these collisions will bewith oxygen, nitrogen, or other nonreactive molecules. A small fractionof the collisions will be with the trace molecules of interest. In thesecases the primary ions will interact with the trace molecules to formsecondary ions. The secondary ions, will have, in general, anappreciable difference in mobility from the primary ions. Theseion-molecule reactions take place in the region near the radioactivefoil cathode as the gaseous sample passes out of the inlet tube 36. Thevolume of inert gas flow from inlet 42 is kept large relative to thesample gas flow from inlet 36 (for example, at least twice as large) toensure that the cell is primarily filled with inert gas. Thus, theionmolecule reaction region is quite limited in volume, but neverthelessthere is sustained ion-molecule reaction in this region.

The ion flux at the first shutter grid 18 will consist of both theprimary ions and possibly several species of secondary ions. A sample ofthis mixed ion population is periodically admitted to the drift regionbetween the first and second shutter grids when the first shutter grid18 is opened by momentarily driving all of the grid wires to the gridaverage potential. The second shutter grid 20 is opened momentarily at apredetermined time after the opening of the first grid. The ions thatpass through the second grid drift toward and are collected by the anodel6, and the resultant output current is integrated over several cyclesto give a measurable current. By scanning the time of opening of thesecond grid relative to the first, a drift time spectrum of the ionpopulation can be obtained in the output and recorded to produce anoutput curve (current vs. drift time) as shown in FIG. 3. This permitsthe various ion species to be separated and identified.

Because of the presence of the inert buffer gas within the ion velocityanalysis region, high resolution is obtained, as can be readily observedin the output curve illustrated in FIG. 3. A total of separate ion peaksis resolved. Interference from moisture in the sample, which heretoforehas tended to blur or obscure desired trace gaspeaks, is eliminated. Theapparent mobilities are no longer a function of trace materialconcentration. Moreover, since the reactive volume is greatly reduced,the effective time constant is also greatly decreased. Effective timeconstants of the order of 20 to 100 milliseconds can be realisticallyachieved.

FIG. 2 illustrates a modified form of Plasma Chromatography cell whichmay be employed in the invention. Parts corresponding to those of FIG. 1have been designated by the same reference numerals with the addition ofa prime. While the sampling grid 18' and the timing grid 20 have beendesignated diagrammatically as single grids, the same dual gridstructures as those previously described are employed. A small, slowstream of sample gas is directed through a small inlet tube 36' whichenters the envelope from the side and exits near the cathode 14 in theregion between the cathode and sampling grid 18. An ionizer 38 (such astritium) is located in the tube 36 near its exit. lens are present inthe sample gas stream from the inlet tube 36' because of primary ionformation and ion-molecule reactions which occur in the sample gas inputtube. The nonreactive buffer gas enters the envelope at 42, and allgases exit from the envelope at 40'.

the gas flow, which may be of the order of 50 centimeters per second,for example. Because of the small active or sample volume of theinvention, the invention is ideally suited for gas chromatographdetector use. For such use with the apparatus of FIG. 1, for example,the effiuent from the gas chromatograph (inert carrier plus trace gas)is inserted into the inlet 36. A separate reactive gas for formation ofprimary ions may also be inserted into the inlet tube.

The invention is not restricted to velocity analysis by means of ionshutter grids, but is also applicable, for example, to analysisinvolving gas flow as a parameter, as set forth in the copendingapplication of Martin J. Cohen, David I. Carroll, and Roger F. Wernlund,filed Nov. 26, 1968, and entitled "Apparatus and Methods for Separating,Detecting, and Measuring Trace Gases." For such application the ions areinjected into an inert gas stream in a duct after formation in aseparate chamber.

While preferred embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that changescan be made in these embodiments without departing from the principlesand spirit of the invention, the scope of which is defined in theappended claims.

The invention claimed is:

l. A method of detecting a trace material in a gaseous sample, whichcomprises reacting said trace material with primary ions to formsecondary ions of said material at a first region, analyzing saidsecondary ions in an inert gaseous medium at a second region, andmaintaining the length of the mean free path of the ions at said regionsvery much smaller than the dimensions of the regions.

2. A method in accordance with claim 1, in which the analyzing of saidions comprises subjecting said ions to a drift field at said secondregion.

3. A method in accordance with claim 1, and in which the analyzing ofsaid ions comprises electrically sampling a portion of the ions at saidsecond region.

4. A method in accordance with claim 3, in which said sampling comprisesadmitting a group of said ions to a first location in said drift fieldand thereafter admitting a portion of said group to a second location insaid drift field for detection.

5. A method in accordance with claim 1, in which the primary ions areformed by ionizing a gaseous carrier of said trace material.

6. A method in accordance with claim 5, in which said regions are in gascommunication and said carrier is supplied to said first region at arate substantially less than that at which said inert gaseous medium issupplied to said second region.

7. A method in accordance with claim 2, in which said inert gaseousmedium is caused to flow through said second region in a directionopposite to the drift of said ions under the influence of said driftfield.

8. Apparatus for detecting the presence of a substance in a gaseoussample, which comprises an envelope, means for forming ions from saidsample at a first region in said envelope by ion-molecule reactions,means for applying a drift field to the ions in said envelope to causethem to drift in a second region, means for producing an electricaloutput from said envelope in accordance with the velocity ofpredetermined ions in said drift field, and means for supplying an inertgas to said second region, said regions being in substantiallyunrestricted gas communication with each other, and the length of themean free path of said ions in said envelope being very much smallerthan the dimensions of said envelope.

9. Apparatus in accordance with claim 8, in which said means for formingions from said sample comprises means for forming primary ions from themolecules of a first gas and for reacting said primary ions with themolecules of said substance to form secondary ions.

10. Apparatus in accordance with claim 8, in which said means forapplying said drift field comprises a pair of spaced electrodes in saidenvelope, one of which is adjacent to said first region and the other ofwhich is adjacent to said second region, said means for forming saidions being adjacent to said one electrode and said means for producingan electrical output comprising said other electrode.

11. Apparatus for detecting the presence of a substance in a gaseoussample, which comprises an envelope having a pair of spaced principalelectrodes therein, one of said electrodes having associated therewithionizing means and the other of said electrodes constituting an outputelectrode, means for establishing an electric field between saidelectrodes, means for introducing a gaseous sample into said envelope atthe region of said one electrode, and means for introducing an inert gasinto the space between said electrodes, said space being substantiallyunrestricted for gas flow between said electrodes and the length of themean free path of ions of said sample in said envelope being very muchsmaller than the dimensions of said envelope.

12. Apparatus in accordance with claim Ill, further comprising outletmeans for withdrawing gas from said envelope.

13. Apparatus in accordance with claim 12, said outlet means beingcloser to said one electrode than to said other electrode.

14. Apparatus in accordance with claim 111, said means for introducingsaid sample into said envelope comprising a pipe terminating in thevicinity of said one electrode.

15. Apparatus in accordance with claim 14, said ionizing means beinglocated within the end portion of said pipe.

l6. Apparatus in accordance with claim 14, said ionizing means beinglocated exteriorly of and adjacent to the end portion of said pipe.

17. Apparatus in accordance with claim lll, further comprising a pair ofion gates arranged in succession between said electrodes.

18. Apparatus in accordance with claim 17, further c0mprising means foropening said ion gates in succession.

19. Apparatus in accordance with claim lll, wherein said means forintroducing said sample comprises means for supplying a trace gas in aninert carrier gas and for introducing a separate reactive gas.

20. Apparatus in accordance with claim 11, said electrodes being locatedadjacent to opposite ends of said envelope, said means for introducingsaid sample and said inert gas into said envelope comprising pipesterminating in said envelope adjacent to said opposite ends.

21. Apparatus in accordance with claim 20, further comprising a gasoutlet pipe leading from said envelope adjacent to said pipe forintroducing said sample.

22. A method in accordance with claim l, in which said secondary ions,together with unreacted primary ions, are analyzed in an inert gaseousmedium at a second region.

2. A method in accordance with claim 1, in which the analyzing of saidions comprises subjecting said ions to a drift field at said secondregion.
 3. A method in accordance with claim 1, and in which theanalyzing of said ions comprises electrically sampling a portion of theions at said second region.
 4. A method in accordance with claim 3, inwhich said sampling comprises admitting a group of said ions to a firstlocation in said drift field and thereaftEr admitting a portion of saidgroup to a second location in said drift field for detection.
 5. Amethod in accordance with claim 1, in which the primary ions are formedby ionizing a gaseous carrier of said trace material.
 6. A method inaccordance with claim 5, in which said regions are in gas communicationand said carrier is supplied to said first region at a ratesubstantially less than that at which said inert gaseous medium issupplied to said second region.
 7. A method in accordance with claim 2,in which said inert gaseous medium is caused to flow through said secondregion in a direction opposite to the drift of said ions under theinfluence of said drift field.
 8. Apparatus for detecting the presenceof a substance in a gaseous sample, which comprises an envelope, meansfor forming ions from said sample at a first region in said envelope byion-molecule reactions, means for applying a drift field to the ions insaid envelope to cause them to drift in a second region, means forproducing an electrical output from said envelope in accordance with thevelocity of predetermined ions in said drift field, and means forsupplying an inert gas to said second region, said regions being insubstantially unrestricted gas communication with each other, and thelength of the mean free path of said ions in said envelope being verymuch smaller than the dimensions of said envelope.
 9. Apparatus inaccordance with claim 8, in which said means for forming ions from saidsample comprises means for forming primary ions from the molecules of afirst gas and for reacting said primary ions with the molecules of saidsubstance to form secondary ions.
 10. Apparatus in accordance with claim8, in which said means for applying said drift field comprises a pair ofspaced electrodes in said envelope, one of which is adjacent to saidfirst region and the other of which is adjacent to said second region,said means for forming said ions being adjacent to said one electrodeand said means for producing an electrical output comprising said otherelectrode.
 11. Apparatus for detecting the presence of a substance in agaseous sample, which comprises an envelope having a pair of spacedprincipal electrodes therein, one of said electrodes having associatedtherewith ionizing means and the other of said electrodes constitutingan output electrode, means for establishing an electric field betweensaid electrodes, means for introducing a gaseous sample into saidenvelope at the region of said one electrode, and means for introducingan inert gas into the space between said electrodes, said space beingsubstantially unrestricted for gas flow between said electrodes and thelength of the mean free path of ions of said sample in said envelopebeing very much smaller than the dimensions of said envelope. 12.Apparatus in accordance with claim 11, further comprising outlet meansfor withdrawing gas from said envelope.
 13. Apparatus in accordance withclaim 12, said outlet means being closer to said one electrode than tosaid other electrode.
 14. Apparatus in accordance with claim 11, saidmeans for introducing said sample into said envelope comprising a pipeterminating in the vicinity of said one electrode.
 15. Apparatus inaccordance with claim 14, said ionizing means being located within theend portion of said pipe.
 16. Apparatus in accordance with claim 14,said ionizing means being located exteriorly of and adjacent to the endportion of said pipe.
 17. Apparatus in accordance with claim 11, furthercomprising a pair of ion gates arranged in succession between saidelectrodes.
 18. Apparatus in accordance with claim 17, furthercomprising means for opening said ion gates in succession.
 19. Apparatusin accordance with claim 11, wherein said means for introducing saidsample comprises means for supplying a trace gas in an inert carrier gasand for introducing a separate reactive gas.
 20. Apparatus in accordancewith claim 11, said electrodes being located Adjacent to opposite endsof said envelope, said means for introducing said sample and said inertgas into said envelope comprising pipes terminating in said envelopeadjacent to said opposite ends.
 21. Apparatus in accordance with claim20, further comprising a gas outlet pipe leading from said envelopeadjacent to said pipe for introducing said sample.
 22. A method inaccordance with claim 1, in which said secondary ions, together withunreacted primary ions, are analyzed in an inert gaseous medium at asecond region.